Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, degrading turbocharger performance and possibly compressor surge. Compressor surge can lead to NVH issues such as undesirable noise from the engine intake system.
To address compressor surge, engine systems may include a compressor bypass system to enable rapid decaying of boost pressure. One example of such a compressor bypass system is shown by Blaiklock et al. in US 2012/0014812. Therein, the compressor bypass system is close-coupled to the compressor housing and includes a valve that allows a portion of the boosted air to be recirculated from the compressor outlet to the compressor inlet. Specifically, in response to an indication of surge, the compressor bypass valve is opened to direct a portion of the air discharged from the compressor to the compressor inlet.
However, the inventors herein have identified potential issues with such an approach. As one example, during the initial phase of a tip-out, some re-boosting of bypass gases may occur in the compressor bypass system of '812, amplifying the compressor temperature. For example, when the throttle is closed and the compressor bypass valve is opened in response to the tip-out, the gas recycled around the compressor continues to get boosted, and therefore heated. This leads to a temperature amplification effect as the heated air continues to be recirculated around the compressor.
In one example, some of the above issues may be addressed by a method for a boosted engine system comprising, in response to an indication of compressor surge, recirculating compressed air from downstream of a charge air cooler and upstream of an intake throttle to a compressor inlet.
As an example, a boosted engine system may include a turbocharger having a compressor driven by a turbine, and a charge air cooler coupled downstream of the compressor for cooling boosted air before delivery to an engine intake. At least a first recirculation passage may be provided for recirculating cooler boosted air from downstream of the charge air cooler to a location upstream of the compressor inlet. Additionally, a second recirculation passage may be provided for recirculating warmer boosted air from upstream of the charge air cooler to the location upstream of the compressor inlet. In one example, each recirculation passage may have a dedicated valve. Alternatively, the two recirculation passages may merge at a position upstream of the compressor inlet, and a common recirculation valve may be used. The recirculation valve may be a continuously variable valve whose position is adjustable to any position from a fully open position to a fully closed position.
In response to an indication of surge, an engine controller may increase an opening of the valve to recirculate cooled boosted air from downstream of the charge air cooler to the compressor inlet. For example, the valve may be maintained in a partially open position during boosted engine operation to improve the margin to surge, and may be shifted towards the fully open position in response to the indication of surge. The opening of the valve may be adjusted based on a desired compressor (or turbine) deceleration speed profile, and/or based on a desired compressor inlet temperature. Further still, the valve position may be adjusted to vary a proportion of cooled boosted air relative to warm boosted air that is recirculated to the compressor inlet, allowing for further compressor inlet temperature control. For example, a proportion of cooler boosted air may be increased and a proportion of warmer boosted air may be correspondingly decreased.
In this way, by recirculating cooler boosted air from downstream of charge air cooler to the compressor inlet when at or near a surge limit, a temperature amplification effect is reduced. In addition, by locating the compressor bypass system post the charge air cooler, and closer to the engine intake throttle, the natural momentum of the air flow can be advantageously used to force flow through the recirculation passage during a throttle closing event (e.g., a tip-out), thereby improving mass flow through the compressor and the margin to surge. Further still, since the margin to surge initially follows a path of increased mass flow through the compressor instead of rapid pressure decay, if a tip-in is requested soon after (e.g., immediately after) the recirculation valve is opened, the higher boost pressure that is already available can be used to build torque to meet driver demand. As such, this improves overall torque response and driveability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.